Cable testing system



Sept. 29, 1931. R. .G. M CURDY CABLE TESTING SYSTEM Filed Dec. 10, 1929 iii {Ni W m uu M;

3 W m MN w M INVENTOR RGJIQ (2W9 ATTORNEY Patented Sept. 29, 1931 UNITED ,imres} r 0mm RALPH G. iyrccunnY; or ENGLEWQOD, NEW ERSEY; assrenoaro amnnrcm TELE- 'VIPHONE nun TnL GnArH'coMI-anY, a conrosarron orNE-w YORK CABLE TEsTINef SYSTEM 1 Application filed membrane-1929. Serial. 110,413,132; 1

This invention relates to cable. testing systems. This application describes an improved method of determining thedegree of unbalance of conductors of a telephone cable, or the like; 4 3 It has been learned that disturbances in. the

circuits of ate-lephone-cable are frequently due to the action of induction on unbalances betweenthe conductors of the cable and other conductive surfaces, which causes the voltage to the sheath of the cable, or'to earth, of the two sides of a particular circuitfto be different. In the ordinary cable the unbalance'of any working circuit is almost purely capacitance. It follows, therefore, that the impor tant unbalances which maybe associated with any pair of conductors in a cable are depend ent upon the capacity between theconductors and associated conductive paths," Thatis to say, the capacity unbalance'inay be. considered as that between the pair 'ofpartic'ular conductors and the surroundingcable sheath, and/or between thepair of conductors and the neighboring conductors of the cable; isnow apparent therefore, that any reduction in the magnitude of these capacity unbalances will give an almost corresponding decrease in the susceptiveness of the cabl'e circuits'to induction from power circuits. 7 j

Efforts have been made in the past to reduce the capacity imbalances contributing to noise by measuring in adjacent cable sectionsthe so-called capaeity unbalance to ground and splicing adjacent lengths of the cable in such away as to minimize this capacity'unbalance, This proces has required a compromise with the splicing procedure intended toininimize imbalances contrlbuting to cross-talk, or else the making of additionaltesting splices ffor diminishing such capacity unbalance to ground ascontributestonoise.

' This so-cal'led capacity unbalance to ground-may"be determined by measuring the capacity unbalance 'ofja particular pair or quad of the cable with respectto the sheath while all of the'other. conductors of the cable are in a floating condition; 'Since the floating pairs or quads practically'assume. sheath potential this determination is the equivalent of the measurement of thesu n 0f all of the direct capacity unbal'ances of the particular pair or quad under .test with respect to the sheath and to all of the other wires. his determination correctly represents the capacity unbalance contributing to noise when only a single pair or quad of the cable is connected to a branch cable or to an open-wire extension which is involvedin exposure to a power circuit or the like. However, this is aicase not frequently encountered in the field and is therefore of little practical importance. 1

In the; usual cabling case either the whole cable is exposed directly to power induction, or a considerable number of pairs. of the cable are connected to an exposed branch cable'or to an exposed open wireextension. In either case there will be a substantial 1mmber of pairs or quads all at aboutthe same potentialwith reference, to the sheath and. to

the remaining pairs or quads. The direct capacity. unbalances among the various pairs or quadswhich are at the same potential, do not, however, contribute to nolse.

, i In order that the splicing of a cable inay be carried'out in such away as to reduce the unbalances contributing to noise, it is necessary that the test procedure shall correctly represent the distributioniof potential which arises from the conditions of exposure against which; protection is desired. It will be apparent that, two cases are of interest. First,

"the one which arises when thecentire cable is of itself involved in inductive exposure, Second, that which consists ,"in connection 1 with the so-called toll entrance cables when aiconnected open-wire or a cable branch is involved inexposureg In the first casefexper ience indicates that the pairs orquaols connected to equipment located at the sameplaces-and included in the same splicing groups, all tend to assume the same potential when thecable is so exposed. The potential difference between the various segregated groupsor layers of the cable, and

between these groups orlayers and the sheath,

are large as compared tozthe potential clifferences within a given.groupi orlayer. vReduction of the 'capacityunbalances contributing to noise, may therefore be accomplished by splicing the adjacent sections ofthe cable accom an in drawin s in which Fi ure 1 shows apparatus'ror the measurement of the direct capacity unbalance ofa pair of con-c ductors with respect to the sheathof the cable, and Fig. 2 shows apparatus for the measurement of thed irect capacity unbal-p I ances of a pair of conductors with respect to anotl'i'e'r group or layer of quads. v

In Fig. 1 of the drawings the reference characters R and R K and K represent the-'arms-of a VVheatston'e bridge used for the measurement of capacity. The arms B and R usually have equal resistances, and the arms K and K may have-variable condens- 'ers each of which may be adjusted in its capacitance to any desired extent;

A generator supplies alternating current of any frequency through a shielded transformer T to the terminals 1 and 2 representing one diagonal of the bridge. An indicating instrument or detector such as a telephone receiver, is de signated D and a single-pole double-throw switch is designat edS. The terminals of the switch, which may be closed by its manipulation, are designated a and Z). The indicating instrument '1)- may re be connected between the terminals 8 and 4;

of the bridge as a diagonal of the bridge, when the switchS is thrown. so as to close the contact a.

The arrangementof Fig. 1 provides means to determine the capacity unbalance-between two oondu'ctorsCi and'C rofa particular quad of the cable and the sheath er the cable designated'L. The conductors O and G are eonneeted'to the terminals 3 and 4 of the bridge as shown, The remaining-conductors C and C of the same quad, frequently known as the mate, as well as the conductors of other quads Q and Q' which'ar'e in the same layer as the one including the conductors C 5 C C and C and th e conductors of the quads of other layers'desig'nated Q and Q and Q and Q' are all connected to the terminal 1 of the bridgethrough an elementof variable resistance designated'R The cable sheath L is connected to the terminal 2 of the bridge and may be considered as grounded.

In the arrangement shown in Fig. 1 of the drawings, all of the resistance of the element R may be out out and the switch S then thrown soas to close its associated contact I). If a sound is heard at the detecting device D it will indicate that the conductor C is at a different potential from the conductors and Q; and the conductors of the quads Q and Q' Q and Q5, and Q and Q This clif of the quads Q and Q' Q and Q' and Q and Q' at the same potential as each of the conductors G and C the variable arm of the element R may be adjusted until no sound is heard in the detectingdevice D. By includin resistance aria, it is possible tolmaiiitaiii each conductor C and (350i the pair under test,- at the same'potential as the other conductors C, and C of the same quad, and the oonductorsiof the remaining quads which are connected to the conductors C and C 7 Theswitch S maythe'n be thrownso as to close its associated contact a. The coirdens'ers K and" K may thenbe adjusted in their'capacit'ance values until'the bridge be comes balanced, and the balanced condition will be indicated by the absence of any'so'und at the indicating device D; If the condensers K and K include calibrated scales to indicate their capacitance values, then these scales will indicate the unbalance of the conductors C and Cg with respect to' the sheath of the cable L,'while the remaining conductors C and C of the same quad and the conductors of the quads Q and Q Q 'and Q, and'Q and Q are maintained' atthe same potential respect to the pair or quad under test." In

this case the sheath L, the conductors of the same quad C and G the conductors of other quads Q and Q2 in the same layer as those of the quad under test, and the conductors of the quads Q and Q' of still otherlayers, are all connected to'the variable arm ofthe element R The testing proc'edureisthe same as already outlined and needlnot be repeated. I

It will be apparent that the bridge arrangement shown in the drawings may be employed for the case of the so-called toll entrance cable which is connected to an exposed open-wire.

Experience here indicates that thepairs of the cable not connectedto theopen-wire will be practically at the potential ofthe'sheath, and that the pairs-which are connected to the exposed open=wire will be nearly all at the same potential whichis diflerent from that of the sheath. The. testing procedure then consists in connecting all ofthe pairs or quads terminatin in the epemwire to the bridge,

that is, to R soth'at these'will be at-thepo- "its tential of the pair or quad, the capacity unbalance of which is being tested. The other pairs or quads not connected to the open-wire may be connected to the terminal 2. The testing procedure will then determine the direct capacity unbalance with respect to the conductors connected to terminal While this invention has been shown in cer tain particular arrangements merely for the purpose of illustration, it will be understood that the principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the claims.

hat is claimed is:

1. The method of determining the degree of unbalance in a telephone cable which consists in measuring the capacity unbalance between two of the conductors of the cable and its sheath and maintaining all of the other conductors of the cable substantially at the same potential as said two conductors.

2. The method of determining the unbalance between certain conductive parts of a telephone cable with respect to other conductive parts of the cable which consists in measuring the capacity unbalances between the first conductive parts with respect to the second conductive parts and maintaining the remaining conductive parts of the cable scope of the appended tween any two of the conductors in anyone of the layers of the cable and any group of other conductors within any other layer, and V at the same potential as the first conductive parts of the cable.

3. A testing system for a telephone cable including a lVheatston-e bridge adaptable for capacity measurement, two conductors of the cable connected respectively to the two terminals representing one diagonal of the bridge, a detecting device connected to the same two terminals of the bridge, and a source of alternating current connected to the other two terminals representing the other diagonal of the bridge, the remaining conductors of the cable being connected to one of said other terminals, the sheath of the cable being connected to the other of said other terminals,

4. A testing system for a cable comprising a Wheatstone bridge having two variable condensive elements as two of the. arms of the bridge, two of the conductors of the cable being connected respectively to two of the terminals of the bridge respresenting' one diagonal, a detecting device connected to the same two terminals of the bridge, a source of alternating current connected to the other two terminals representing the other diagonal of the bridge, and a variable resistance connecting the remaining conductors of the cable to one of the latter terminals of the bridge, the sheath of the, cable being connected to the other of the latter terminals of the bridge.

5. The method of determining the degree of unbalance in a telephone cable, which consists in measuring the capacity unbalance be- 

